1. Field of the Invention
The invention pertains to the field of solid fuel combustion. More particularly, the invention pertains to sustained burning of explosible biomass powder with on/off control.
2. Description of Related Art
The present invention concerns processes, methods, devices, and systems that, taken separately and together, allow for the processing of biomass and other solid fuel materials into an explosible powder and the combustion of the materials for a direct conversion into energy to heat or perform work. This disclosure describes the harnessing of long-feared dust explosions and operating new solid fuel burners to accomplish a unique energy conversion process.
The present invention is largely based upon the application and new integration of some advanced yet elegantly simple principles, portions of which exist unconnected in various bodies of knowledge in the fields of fluid mechanics, physics, kinetics, industrial power plant process design, and combustion theory. This technology will soon gain integrated global scientific community attention and be applied in the engineering of fuel source production, distribution, combustion burner design, heating, and other energy conversion applications.
A thorough discussion of the invention's background and practical and theoretical bases is presented to convey the uniqueness of the invention, the scope of its various embodiments and variations, and how it may be practiced. The present disclosure shows how prior art attempts to utilize powder in fuels have come up short, failing to unveil a practical and complete picture of the methods and processes, which will soon be established as a new body of knowledge and practice, becoming an affordable and practical alternative to America's ever growing need for renewable energy.
Before delving into the prior art, it is important to summarize key points about this new art and what performance and benefits can be expected from its implementation. A burner of the present invention preferably has numerous features: instant cold start ON-OFF control; stable combustion the moment the powder-air mix is ignited; use in either vertical and horizontal modes; burning solid fuel in a single-phase mode as if it were a vaporized liquid or gas; completeness of combustion within the burner housing itself, rather than in a large high temperature furnace reactor; an ultra-short particle residence time requirement; burning substantially explosible powders; recycle consuming with self-contained management of initially unburned particles; and smaller and simpler than prior art solid fuel systems. The burner and fuel in combination are important to operation of a burner of the present invention, as are the burner itself, the type and quality of fuel, and integration with a Positive Displacement Powder Dispersion (PDPD).
A major point in our disclosure is the surprising revelation that a solid may be heterogeneously combusted in a gas in a method that differs very little from a true single phase regime, yet differs greatly from traditional combustion practices over the years which continued to rely on two-phase principles of a stirred reactor. This topic will be introduced in the review of prior art next, and explained in-depth later in fluid mechanics terminology with reference to theory.
What has been the thinking, goals, and focus for design of burners, furnaces, and fuels over the last three to five decades, both in large power plant burner, furnace and heat recovery design and fuel selection? It is apparent, after review of representative literature written during the last half century, that the basics of furnace design assumptions practiced in the mid-twentieth century still control mainstream thinking.
Residential and small commercial heating furnace design assumptions have remained similarly bound and influenced by larger power plant concepts, except for changes in two significant areas. First, process control and energy saving design improvements have resulted in increased efficiency of heat recovery from small to large furnaces. Today, latent heat is extracted from hot flue gases with efficiencies in the low 90th percentile normally. Second, using technology formerly only affordable in power plant furnace systems, these smaller furnaces and boilers are beginning to experience technology additions to reduce airborne post combustion pollutants, since it has not been cost-effective on a per BTU basis or mandated outside the power plant.
Practicing new techniques of air pollution abatement have produced major strides forward by reducing, removing, and cleaning various pollutants from power plant and furnace flue exhausts. Increased use of biomass based fuels for co-firing with fossil fuels has further reduced stack emission levels. Ultra-clean coal, may soon become an affordable option for the residential and commercial users, but due to processing costs, has yet to become economically attractive for large coal fired generating stations.
The use of biomass for heating or transportation is often limited by our experiences as well, both on individual and governmental levels. We tend to think that alternatives to fuel oil must be liquids and fuels must transport and pump like liquids. Likewise, supplements to gasoline must be liquid, except for wood gas and the hope for hydrogen.
Relevant Combustion History, Fuels, and Practices
There are several conditions which must exist simultaneously to achieve complete combustion, known in the industry as the “Three T's”. The fuel mixture must be 1) in an environment of adequately high Temperature; 2) for a sufficiently long enough Time; 3) with reasonably Turbulent mixing conditions to provide proper oxidation to complete fuel combustion in the Space allowed (see C. E. Baukal, Jr., ed., The John Zink Combustion Handbook), and that “Space” is known by a variety of names in the industry such as a furnace, combustion chamber, boiler, firebox, and process heater (vertical cylindrical, cabin style and reactors), all of which are large chambers or vessels emulating an “ideal mixed reactor”. It is important to remember that the primary method of heat transfer to the fuel particles in such large furnaces is by radiation rather than conduction from particle to gas as we employ.
Even back in 1950, furnace and burner design was driven by the goal of attaining “ideally mixed reactions” as it states in the Plant Engineering Handbook (W. Staniar, ed.), incorporated by reference herein. Design of a burner of the present invention must deal with the bulk these same criteria, but is not constrained to use of a model of the downsized power plant for furnace design, which requires a hot, radiating refractory and its inherent large size. As a benefit, our burners can start up cold, and operate with ON/OFF control, unlike coal fired furnaces burning pulverized coal, which take hours to startup and shut down:
“The development of pulverized-coal firing for purposes of steam generation has been due, in large measure, to a better understanding of furnace design requirements. Uniform distribution of fuel and air to the furnace is also of prime importance. Turbulence provides the means for effective distribution and speed(s) ignition of the incoming fuel and promotes rapid combustion by continually making available the free oxygen needed by the ignited combustible matter. These requirements are the governing factors in burner selection and application.”
“The selection [of a firing method] for any given installation is governed by a number of variables, of which the principal ones are size, shape, and volume of furnace available to develop the desired capacity. Furnace dimensions establish the maximum length of flame travel available . . . . Quantity of coal to be burned, as well as its volatile matter and sulfur content, fusion temperature of ash, and fineness of pulverization will influence not only method of firing and type of wall construction to be used but also the method of ash disposal.”
“Each of the firing methods [vertical (downward) firing; horizontal turbulent firing; and tangential firing] requires a different burner design because of the variations in the manner in which air and coal are mixed to produce efficient and complete combustion. Fundamentally, however, all burner designs must be such that the air and coal are supplied to the furnace so as to provide stable and prompt ignition; positive adjustment and control of ignition point and flame shape, completeness of combustion; uniform distribution of excess air, temperature, and gas flow at furnace outlet; freedom from localized slag deposits; protection against overheating, internal fires, and excessive wear in the burner; and accessibility for adjustment and replacement of parts.”
Coal has been burned both in crushed and pulverized forms for over half a century. There are many variables that interplay including moisture, percent volatiles, ash, and BTU value for given types of coal and the type of furnace. For example, a high percent volatiles can cause heating value loss or excess smoking issues for stoker-fired plants, if the specific furnace has inadequate space and time to mix the volatile gases with air and completely combust them. A requirement for an upper limit on volatiles is a typical solution for specific types of furnaces.
Conversely, when firing pulverized coal, it is important “to set a lower limit (for percent volatiles) in order to maintain flame propagation, particularly in completely water-cooled furnaces” according to the Plant Engineering Handbook, page 373:
“Solid fuels, when burned in suspension, should contain an appreciable quantity of extremely fine dust so as to ensure prompt ignition. The amount of coarser material must be minimized if best combustion results are to be obtained.”
“The fineness to which coals should be pulverized will depend on many factors. Caking coals (sulfur containing bituminous coal, coking coal, forms a fused heavy crust at the surface), when exposed to furnace temperature, will swell and form lightweight, porous coke particles. They may float out of the furnace before they are completely burned. As a result, carbon loss will be high unless pulverization is very fine. Free-burning coals (contains no sulfur and does not cake), on the other hand, do not require the same degree of fineness because the swelling characteristic is absent.”
“High-volatile coals ignite more readily than those with low volatile content. Therefore, they do not require the same degree of fine pulverization. With the exception of anthracite (called stone coal), however, the low-volatile coals are softer and may be said to have a higher grindability.”
“Some large furnaces may operate satisfactorily on high-volatile coal as coarse as 65 percent minus 200 mesh. Small water-cooled furnaces, using low-volatile coal, may require a fineness of 85 percent minimum through 200 mesh. Other influencing factors are burner and furnace design, disposition of furnace volume, length of flame travel, furnace temperature, and load characteristics. In general, however, small furnaces require finer pulverization than large ones.”
“The fineness of the product is usually expressed by the percentage of dust that will pass a sieve with specific size openings. For testing pulverized coal the most commonly used sieves are the 50-mesh sieve (210 microns) for determining the oversize and the 200-mesh sieve (74 microns) for determining the fine dust.”
In summary, the use of pulverized coal in furnaces is most beneficial for ignition, where an “appreciable quantity of extremely fine dust” is utilized. Pulverized coal offers benefits for handling with many types of stokers but is a known detriment and avoided for caking types of coal. Coal raw material composition and resulting combustion issues can be compensated for by adjusting the particle size to be finer or coarser. The percentage of volatiles is similarly employed as a trade-off, with a higher volatile percentage enabling a distribution of larger particle size coal to be used.
As evidenced by the art practices, pulverized coal particle size is used to address fuel handling and coal type composition issues. By relying on the % volatiles consumed in multi-phase combustion combined with large reactor size and residence time common to all furnaces, it is clear that there is no teaching in the art for using only substantially explosible coal powder as a feed stock with a cold, small and low-speed burner design. The main value of having a portion of the overall pulverized size distribution well below 200 microns is for reliable ignition and fast burn only. Further in-depth fluid mechanics comparison of the differences between of our combustion regime and coal power plant furnace techniques may be found later in this disclosure.
In traditional furnaces used for steam generation in power plants, whether they are fed by coarse crushed coal or fine pulverized coal, much of the actual combustion takes place over time inside the furnace's large volume. Typically, multiple burners are used to “fire” into the radiation filled furnace cavity reaction chamber, where much of the combustion is completed and heat energy is released for subsequent exchange.
Biomass, Wood and Hog Fuel Combustion
Sources of biomass have been used sporadically in localized developments to convert accumulating “bio-scrap” for recovery of some of its energy content and to “dispose” of this otherwise waste product. The pulp and paper industry and affiliated sawmill industry are leading examples. The following gives perspective to the supply.
Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply, (R. D. Perlack et al.) sponsored by the U.S. Departments of Energy and Agriculture in 2005 offers two significant quotes from the Executive Summary regarding the availability of biomass as a fuel source:
“This study found that the combined forest and agriculture land resources have the potential of sustainably supplying much more than one-third of the nation's current petroleum consumption.” And regarding development of a vertical industry of supply: “In the context of the time required to scale up to a large-scale biorefinery industry, an annual biomass supply of more than 1.3 billion dry tons can be accomplished with relatively modest changes in land use and agricultural and forestry practices.”
Large “powder burners” from either Petrokraft or the VTS Powder burner are utilized in Sweden and in Europe according to a 2004 doctoral thesis written by Susanne Paulrud, “Upgraded Biofuels—Effects of Quality on Processing, Handling, Characteristics, Combustion and Ash melting”. This technique is typically applied to large-scale heating plants over a megawatt. The fuel for these burners is “finely milled wood powder or finely milled pellets.” Wood powder analyzed by sieve and laser methods shows percentages of explosible particles ranging from 3% to about 46%, far too low to operate in the explosible mode. These burners utilize classic swirl for containment and recirculation mixing, but “aerodynamics and stoichiometry can make it difficult to achieve stable ignition and good burnout”. Even with the finest particles, predicted particle traces show distinct zones for evaporation, boiling, and devolatilization before char burnout, indicative of two phase combustion. This fact plus the large burner airflow designs and particle distributions used confirm no capability of operation mimicking a single phase combustion regime.
Large burners such as the German burner utilized in the Canadian system by Alternative Green Energy Systems Inc. (AGES), likewise consume wood particles, sawdust and what they describe as powder as evidenced by the complexity, orientation, and ash concerns of their combustion equipment. This advanced system, however, was clearly not designed for exclusive use of a “substantially explosible” biomass wood powder.
U.S. Pat. No. 4,532,873, “SUSPENSION FIRING OF HOG FUEL, OTHER BIOMASS OR PEAT”, issued in 1985 to Rivers et al., is an excellent example of the previous and current art when it comes to the direct burning of various types of biomass for heat recovery, in this case in a water-wall boiler.
While this hog fuel biomass burning system may initially seem very similar to our disclosure, detailed examination will make absolutely clear that this entire system operates using a totally different combustion regime and substantially different operating principles burner hardware, and fluid mechanic processes in its two-phase operation.
The patent states that the fines portion is an ignition source, imparting stability to the flame, and that “the presence of the fines portion is the heart of the invention” as it simply “eliminates the requirement for running with supplemental oil . . . . Hog fuels must be substantially reduced in size to provide an ignition energy source”. The large particle size distribution curve of FIG. 1a depicts a typical hog fuel non-explosible particle size distribution, compared to an explosible powder particle size curve on the left.
The stated stability of their two-phase combustion regime has only a 2.5:1 turndown ratio compared to our 10:1 ratio, and their burner cannot tolerate cold secondary air unlike a burner of the present invention. The process by Rivers, et al. is stated to work “for all furnace configurations, kilns and the like, but is most particularly suitable for use with water wall furnaces and boilers”. It relies on fines to initiate and stabilize the combustion and radiant heat transfer from a hot furnace to complete it, especially when, large and oversized non-explosible particles are concerned.
This hog fuel burner system requires a distribution with particle sizes much larger than ours, allowing for up to an estimated 75% of the particles outside the explosible range (“15-85% less than 150 microns”) and “65 to 100% less than 1000 microns”, meaning 35% could be larger than 1 millimeter (1000 microns), a size that is 4 to 5 times the boundary between explosible and non-explosible wood powders.
Even the slightly narrower region claimed by Rivers, et al. (“at least 60% by weight of the particles are finer than about 1000 microns”) allows for a significant portion of non-explosible particles. The statement “A fines portion including at least 15% by weight less than 150 microns was found suitable” clarifies that there is no requirement for significant or substantial use of “fines”. The hog fuel burner does not operate in what we call the explosible range, a term they never use. Explosiblity is a phenomena they only understood from a standard industry standpoint, for they were afraid of dust explosions like the rest of industry, as made exceedingly clear by this last clarifying statement. “Fuels much finer than 85% less than 150 microns are likely to be too ‘dusty’ increasing dust explosion hazards and otherwise requiring an excess of pulverizing power to produce.”
The present disclosure focuses on combustion of substantially explosible mixtures. Other art, including the hog fuel patent just described and co-firing designs, specify the use of a distribution of a mixed particle size fuel, often called “powder”. Only some component segments of the broad, larger fuel particle size distribution are “fine powders” that may, only when used alone, actually be explosible.
However, these “fine powder” portions are subsets of a much wider and essentially non-explosible fuel particle size distribution such as shown in FIG. 1a, and are utilized at best simply as a quick and easily burnable ignition and combustion maintenance energy source. This small fraction has the stated primary purpose to “sustain combustion” of larger particles and chunks, the major portion of their fuel size distribution lying outside of the explosible range, the region which we disclose, claim, and prefer.
In the present disclosure, all of the fuel performs the functions of ignition and combustion temperature maintenance, not simply a portion nor even a significant portion of the overall fuel composite particle size mixture. Substantially all of the fuel has the job of ignition and heating of its neighbors in the entire mixture, even “less burnable” agglomerated clumps or occasional, longer high aspect ratio non-explosible particles having explosible diameters found in the fuel due to manufacturing sieving/separation imperfection.
In a case study begun in 1995, and entitled “Co-Combustion of Biomass in Pulverised Coal-Fired Boilers in the Netherlands” (M. L. Beekes et al.), co-combustion of pulverized wood with pulverized coal was studied at the Gelderland power station. Waste wood was used in a coal-fired boiler in a pulverized mode, as it “has the advantage of being a very dry and fine fraction material that is uniform, easy to handle, and with high energy content that can be burned much like oil or gas”. The 635-MWe coal-fired production began operation in 1981, and in the mid to late 1980's was upgraded with flue gas desulphurization, NOx reduction, and electrostatic fly ash filters. Using four burners of 20 MWe each, this bio-scrap can provide about 12.5% of the operating energy input.
Wood chip up to 3 cm in size were reduced by a hammermill at the plant to a maximum particle size of 4 mm. The particles were sieved and divided and further separated using a dust collector. The particle size distribution of the wood powder is given as 90% less than 800 μm (a coarse 20 mesh), 99% less than 1000 μm, and 100% less than 1500 μm, with a moisture content of less than 8% by weight. With the material dependent dividing line between explosible and non-explosible powder particle size for wood residing in the neighborhood of 200+/− microns, it is obvious that a significant portion of the particles, likely well over 50%, are not explosible, meaning their combustion process is different from ours.
Combustion took place inside a boiler furnace built in the following burner configuration: “Four special wood burners with a capacity of 20 MWth each are mounted in the side walls of the boiler (two on each side) below the lowest rows of the existing 36 coal burners. There are 3 rows of 6 coal burners in the front and back walls. The coal burners can also be used for burning oil and therefore the combination wood powder/oil is theoretically possible.”
Use of large furnaces as high temperature reactors allowed larger particles as a substantial portion of the entire fuel stream to be utilized, as ignition happens inside the furnace through radiation, not particle-to-gas conduction, characteristic of our single-phase appearing combustion regime.
The “Safety Precautions” section of the report combined with their operating particle size specification and use with a large furnace makes it abundantly clear that this system was not operating in the explosible range and therefore not mimicking a single-phase regime as we practice, disclose, and claim. A byproduct of their “ . . . micronizing process creates wood dust particles that pose a possible hazard for dust explosions. Therefore [the following] safety precautions must be taken . . . . ” Like other recent prior art uses of biomass scrap for industrial energy conversion, they too were unaware of the potential to operate in the combustion regime of the present invention.
In the North American wood products and pellet industries today, it is common to see large cyclonic burners such as units manufactured by Onix used for chip and coarse sawdust drying. These Webb Burners™ are costly and large in size to insure adequate residence time for particle char burnout. Suspension burners are taking over most installations, as they offer efficiency gains of 25% through stat gas recycle plus considerably less maintenance. These large burners are designed for large particle fuel, and therefore do not operate using the principles of this disclosure.
The limitations of the prior art establish the need for systems and approaches for the conversion of biomass and other solid fuel powders directly into energy by such means and methods to afford ON-OFF control of clean, dependable, and efficient combustion.